Measuring the concentration of substances, particularly in the presence of other, confounding substances, is important in many fields, and especially in medical diagnosis. For example, the measurement of glucose in body fluids, such as blood, is crucial to the effective treatment of diabetes.
Diabetic therapy typically involves two types of insulin treatment: basal, and meal-time. Basal insulin refers to continuous, e.g. time-released insulin, often taken before bed. Meal-time insulin treatment provides additional doses of faster acting insulin to regulate fluctuations in blood glucose caused by a variety of factors, including the metabolization of sugars and carbohydrates. Proper regulation of blood glucose fluctuations requires accurate measurement of the concentration of glucose in the blood. Failure to do so can produce extreme complications, including blindness and loss of circulation in the extremities, which can ultimately deprive the diabetic of use of his or her fingers, hands, feet, etc.
Multiple methods are known for measuring the concentration of analytes in a blood sample, such as, for example, glucose. Such methods typically fall into one of two categories: optical methods and electrochemical methods. Optical methods generally involve spectroscopy to observe the spectrum shift in the fluid caused by concentration of the analyte, typically in conjunction with a reagent that produces a known color when combined with the analyte. Electrochemical methods generally rely upon the correlation between the current response of a blood sample and the concentration of the analyte, typically in conjunction with a reagent that produces charge-carriers when combined with the analyte. See, for example, U.S. Pat. No. 4,919,770 to Preidel, et al., and U.S. Pat. No. 6,054,039 to Shieh, which are hereby incorporated in their entireties.
Optical systems have rapidly lost popularity to the electrochemical systems, largely due to the fact that the blood sample must be inserted into the meter itself (into the internal optics block), thereby coming into direct contact with the meter itself. This required a thorough cleaning of the meter internal and external surfaces between uses, in order to prevent contamination of a subsequent sample and to allow a single meter to be safely used on multiple patients in a hospital setting or in a doctor's office without undue bio-risk. In electrochemical devices, the sample chamber is typically placed in a disposable test strip, which is inserted at one end into the meter. This way, the blood sample never makes contact with the meter.
An important confounding variable in electrochemical blood glucose testing is the change in the concentration of the reaction product over time. For example, in strips employing a dry reagent, initially, the reagent on the strip reacts at an accelerating pace, as it becomes wetted. Subsequently, the pace of reaction drops off, as the concentration of the blood glucose in the neighborhood of the reagent drops due to reaction. The concentration of the product in the neighborhood of the reagent initially increases as it is generated by the reaction, but if the reagent is exhausted, will subsequently decrease, as the product diffuses into the rest of the sample. In some prior art systems, the time variation is accounted for by letting the reaction run to completion. However, this method is undesirable because it is very slow. More recent systems have dealt with the time variation by calibrating the measurement to the period between contact of the sample with the reagent and the point of measurement.
However, this method poses a different problem, since it requires that the test strip be inserted into the meter before it is dosed. As a consequence, it requires far more dexterity to successfully dose the strip in such systems, since the meter and strip together are far larger and more cumbersome than the strip by itself. This is especially problematic since diabetics, who are the primary users of blood glucose measuring systems, often suffer from a loss of both fine and coarse motor control. It is well-established in the field of ergonomics that fine motor control (dexterity) is best achieved while attempting to capture a small droplet of blood, derived from penetration of the patient's skin, when a small device such as a test strip is used, in contrast to the combination of the much larger test meter with a test strip inserted therein. In the professional setting (bedside testing), the professional conducting the test often lacks a stable work surface as an aid while conducting the test procedure. In such cases, bringing a large apparatus into contact with a small droplet of blood without the aid of a stable surface is challenging.
Thus, a system and method are needed that accurately measure blood glucose, using a test strip which does not need to be inserted into the meter prior to dosing, and a meter that does not require direct contact with the sample to make the measurement. The present invention is directed to this need, among others.